There is a need for sustainable fuels for diesel engines and fuels containing particles will function as a fuel in diesel engines. Some microalgae such as Chlorella vulgaris are unicellular and 5-10 mum in size, which is suitable for combining in an emulsion fuel. An emulsion consisting of transesterified rapeseed oil, a surfactant and a slurry of C. vulgaris was used as a fuel in an unmodified single cylinder diesel engine. The fuel consumption and emissions of this fuel was determined and although the carbon monoxide levels were higher the NOx emission was lower than that of diesel. (C) 2003 Elsevier Inc. All rights reserved.